Kendrick dew heater controller

Hi,
My Kendrick dew heater controller (model 4) blew up yesterday. The housing
of the TLC555 timer chip had literally blown up. Same thing happended after
I replaced the 555. The MOSFET and diodes look ok from what I can measure.
All other components look visually ok. BTW, what type of component are VR1
and J4?
Anyone who knows the circuit well enough to give me some guidance on which
components to check?
Thanks
Dirk